Ah. The problem is that your step size is negative and PsychoPy isn’t expecting that. It checks the maximum value after adding a step and checks the minimum value after subtracting a step. You’re “adding” something that’s causing the value of the staircase to go down, which is why PsychoPy’s confused.
We could change the staircase to handle a world in which steps “up” result in values going down, but I’m not sure what else that will break. Might be better if we raise an error if you try to set a step to be negative.